1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a method of repairing defects in contact printed electrical circuits using electrohydrodynamic (EHD) printing.
2. Description of the Related Art
As noted in Wikipedia, printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment or other low-cost equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography and inkjet. Electrically functional electronic or optical inks are deposited on the substrate, creating for example, active or passive devices, such as thin film transistors or resistors. These processes can utilize any liquid phase material, including, but not limited to, solutions, mixtures, and dispersions containing organic semiconductors, inorganic semiconductors, organic dielectrics, inorganic dielectrics, metallic conductors, oxide conductors, organic conductors, nanowires, nanoparticles, nanotubes, and nanotubes.
The attraction of printing technology for the fabrication of electronics mainly results from the possibility of preparing stacks of micro-structured layers (and thereby thin-film devices) over large areas in simpler and cost-effective way, as compared to conventional electronics. Also, the ability to implement new or improved functionalities (e.g. mechanical flexibility) plays a role.
Organic field-effect transistors and integrated circuits can be prepared completely by means of mass-printing methods. The selection of print methods for the different layers is determined by dimensional requirements and the properties of printed materials, as well as economic and technical considerations of the final printed products. Optimal resolution of these considerations typically results in a combination of several print methods for the fabrications of the devices, as opposed to a single method.
Printed electronics permits the use of flexible substrates, which lowers production costs and allows fabrication of mechanically flexible circuits. While inkjet, aerosol-jet, and screen printing are used to pattern ink onto rigid substrates like glass and silicon, mass-printing methods nearly exclusively use flexible foil, polymers and paper.
Other methods with similarities to printing, among them micro contact printing and nano-imprint lithography, are of interest. Contact printing technologies of interest include flexography, gravure, screen printing, offset lithography, and variations of each of these techniques. In each of these cases a pattern is generated on a roller, stamp, plate, or screen prior to the printing process. Ink is applied to the pattern carrier and is then transferred to a substrate by the printing process. Here, micron and nanometer-sized layers are prepared by methods similar to stamping with soft and hard forms. Often the actual structures are prepared subtractively, e.g. by deposition of etch masks or by lift-off processes. For example, electrodes for OFETs can be prepared in this manner. Sporadically pad printing is used in a similar manner. Occasionally so-called transfer methods, where solid layers are transferred from a carrier to the substrate, are considered printed electronics.
The ink materials used must be available in liquid form, for solution, dispersion, or suspension. Additionally, they have varying functionality, to serve as conductors, semiconductors, dielectrics, or insulators. Metal inks are also commonly used in printed electronics for reasons of improved conductivity and potential for surface functionality, as compared to their organic counterparts. Silver, gold, and copper nanoparticle inks are used with all of the printing processes described above.
High throughput and fine feature reproduction capabilities make contact printing technologies attractive in printed electronics manufacturing. However, as with any manufacturing technique, defects can occur during the printing process. Defects in the pattern transfer could be expected from many sources. These sources may include contamination of the substrate, contamination of the ink, contamination of the pattern carrier, substrate surface imperfections, ink dispense, localized disturbances in substrate surface energy, and substrate surface roughness to name only a few. These defects may be reproduced over many print cycles or may be limited to a single printed pattern.
Defects that create a discontinuity in metal lines result in open circuit points, for example, that have a dramatic effect on the yield of the printed circuit. Also, due to the repetitive nature of contact printing techniques these defects can be reproduced many times throughout the printing cycle. It is difficult to imagine a contact printing process that would completely eliminate defects or that could identify, as well as repair, printed defects in a metal line.
It would be advantageous if defects in contact printed layers of printed electronics circuits could be repaired in real-time in a range of resolutions.